Cyclic esters, including cyclic esters of the general formula ##STR1## wherein R.sub.1, R.sub.2, R.sub.3, and R.sub.4 can be either hydrogen or an aliphatic or substituted or unsubstituted aliphatic or aryl hydrocarbon having from 1 to about 10 carbon atoms, are a useful class of compounds that can be polymerized into polymeric materials. Such polymeric materials are particularly useful in the preparation of biodegradable plastic materials and plastic materials which dissolve when used in medical applications. Polymers made from the polymerization of such cyclic esters as lactide are particularly useful because they can be degraded over time by water hydrolysis under most environmental conditions. The resulting lactic acid units or oligomers of lactic acid are then readily taken up by organisms in the environment and converted to carbon dioxide and water. Cyclic esters are also useful as plasticizers and intermediates for production of surface-active agents and plasticizers.
In accordance with prior practice, the desired cyclic esters were prepared by first condensing hydroxy acids, typically .alpha.-hydroxy acids, to an oligomeric prepolymer of relative high molecular weight. The prepolymer was then depolymerized at high temperature and low pressure in a heated, evacuated reactor to a crude cyclic ester. Extensive purification processes were required to obtain cyclic esters of requisite purity sufficient to provide polymers of desired molecular weight.
The production of a cyclic ester from an oligomeric .alpha.-hydroxy acid prepolymer is sometimes referred to as a back-biting reaction since it involves the gradual removal of .alpha.-hydroxy acid components from the tail ends of the polymer to form the cyclic ester as illustrated below with reference to a lactic acid polymer. ##STR2##
U.S. Pat. No. 4,727,163 to Bellis is directed to a process which includes first making a prepolymer comprising a block polymer which includes a thermally stable polyether core with an .alpha.-hydroxy acid or its ester polymerized onto the core. Upon heating under vacuum conditions, the chain ends of the .alpha.-hydroxy acids are thermally degraded to form a cyclic ester which can be condensed under vacuum.
U.S. Pat. No. 4,835,293 to Bhatia is directed to a back-biting process which includes the use of an inert gas sweep which permits the process to be operated at or above atmospheric pressure. The inert gas intimately contacts the prepolymer, which is in the liquid phase, so as to create a large interfacial area between the prepolymer and the inert gas to vaporize the cyclic ester and sweep the vapors out of the reactor for subsequent recovery and purification.
As illustrated above, and as discussed in the referenced Bellis and Bhatia patents, back-biting depolymerization of an .alpha.-hydroxy acid can result in the production of a cyclic ester. However, the back-biting reaction is typically a slow one, and a batch operation which extends over significant time and which results in an undesirable high molecular weight byproduct heel which must be disposed of and the cyclic ester product must be separated from noxious discolored pyrolysis products.